Photometric Determination of Bromate in Water and Drinking Water with 3,3'-Dimethylnaphthidine and Iodide
1. Introduction
Since bromate has a potentially carcinogenic effect when ingested orally, a limit of 0.01 mg/L has been set in the currently applicable drinking-water regulations like EU drinking water directive1 or WHO drinking water guidelines.2 The determination of the concentration of bromate is correspondingly a prerequisite measure for the elimination of risks to consumers’ health.
Bromate can arise as a by-product of the ozonation of bromide-containing water depending on the conditions (pH, temperature, duration) prevalent at the treatment site. On account of the swift disinfection result that it provides, ozonation is still a method preferentially used in the treatment of water. It is also recognized that by the disinfection of drinking water with sodium hypochlorite, bromate can be found because there are trace impurities in the sodium hypochlorite solutions. If bromate is a possible by-product of the treatment, the process manager should frequently monitor the bromate content.
The bromate concentration in drinking water can be determined, for example, by means of ion chromatography in conjunction with conductivity measurement and suppressions technique. This method is described inter alia in ISO 15061:2001,3 which is also a constituent part of the “German standard method for water, wastewater, and sludge investigation”. Furthermore, the measurement may be made using LC/ICP-MS. As these methods require access to lab infrastructure and investment into and training for complex instrumentation, we provide photometry as an alternative for on-site analysis in this application note.4
2. Experimental Method
This application note describes the determination of bromate in drinking water using 3,3’-dimethylnaphthidine as a sensing dye in photometry. Bromate oxidizes iodide to iodine in the acidic media provided. This iodine reacts with 3,3’-dimethylnaphthidine to form a pink product which can be determined photometrically.
The methods are preprogrammed on the corresponding Spectroquant® Prove photometers. To eliminate the impact of quality changes in the reagents you use, it is strongly recommended to perform regular AQA checks with appropriate reference standards. Please check the Spectroquant® offering of ready-to-use bromate solutions for AQA purposes.
There are two methods available. The Ultra-Low Range (ULR) method requires an additional evaporation step, which may increase the sensitivity by one order of magnitude, but also requires additional handling steps which are also a possible source of additional imprecision.
To make a good choice for your bromate determination, please:
- Select the right method based on your requirements (accuracy, available instrumentation, and lab infrastructure) from the overview in chapter 2.1.
- Check, if your matrix allows for robust determination, e.g., if the given content of interfering substances is below the threshold for your method provided in chapter 9
- Check, if your analytical process allows for temperature control within the maximum variation you are willing to accept. Information on the impact of temperature changes is provided in chapter 10.
2. 1. Measuring Range |
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2.2. Sample Material
Drinking and mineral water, demineralized water, purified water (reverse osmosis)
Before analysis, please check for possible interferences (see chapter 9 “Influences of foreign substances”).
3. REAGENTS, INSTRUMENTS, AND MATERIALS
3.1. Reagents
- 3,3‘-Dimethylnaphthidine (1.03122)
- Acetic acid 100% for analysis (1.00063)
- Ethanol abs. for analysis (1.00983)
- Potassium iodide for analysis (1.05043)
- Perchloric acid 70–72 % for analysis (1.00519)
- Water for analysis (1.16754)
Optional:
Analytical quality assurance or user-defined calibration:
- Bromate Standard Solution, CRM, 0.0100 mg/L BrO3- (1.33006)
- Bromate Standard Solution, CRM, 0.1000 mg/L BrO3- (1.33007)
- Potassium bromate for analysis ACS, ISO, Reag, Ph Eur 99.8% (1.04912)
- Potassium bromate solution for 1000 mL; c(KBrO3) = 1/60 mol/L (0.1 N) Titrisol® (1.09925)
Cleaning of the glassware:
3.2. Instruments
For the bromate measurement, one of the following Spectroquant® photometers is necessary:
- Spectroquant® UV/VIS Spectrophotometer Prove 600 Plus (173028)
- Spectroquant® UV/VIS Spectrophotometer Prove 300 Plus (173027)
- Spectroquant® VIS Spectrophotometer Prove 100 Plus (173026)
Note: Also, legacy Spectroquant® Prove 100/300/600 instruments are suitable.
Software for data maintenance
The Spectroquant® Prove Connect to LIMS software package provides an easy way to transfer your data into a preexisting LIMS system. This software can be purchased under:
- Prove Connect to LIMS (Y11086)
3.3. Materials
- Rectangular cell 100 mm (1.74011) or
- Rectangular cells 50 mm (1.14944)
- Syringe filter glass fiber (SLAP02550)
- Syringe filter 0.20 μm (SLLGM25NS)
- Flat-bottomed long tubes with screw caps - for measurements in 100 mm cells (1.14901) or
- Empty cells with screw caps 16 mm - for measurements in 50 mm cells (1.14724)
- Standard laboratory glass equipment (e. g. glass beakers, 400 mL tall beakers, 25 mL volumetric flasks, Erlenmeyer flasks, graduated cylinders)
- Pipettes with high accuracy, e.g., piston-stroke pipettes
- Heating plate and boiling granules or
- Magnetic stirrer with heating and stirring rods
Optional, if the sample solution is not clear after evaporation
- Millex®-LCR Syringe Filter, Hydrophilic PTFE, Non-sterile (SLCR025NB) or Fluted filter (radius max. 4 cm)
3.4. Preparation of reagents
- Pre-treatment of vessels and glassware:
Used vessels and glassware must be clean and free from surfactant residues or similar substances. Pre-treat the vessels and glassware with a mixture of isopropyl alcohol and hydrochloric acid, if necessary. Subsequently, rinse thoroughly with distilled water.
Prepare the isopropyl alcohol/hydrochloric acid mixture by placing 3 parts isopropyl alcohol in a glass beaker and slowly adding 1 part of hydrochloric acid 25%. Follow the respective safety regulations! - Preparation of an acetic acid/ethanol mixture [1 + 1]:
In a graduated cylinder, measure 25 ml acetic acid 100 % and transfer to a 50-ml Erlenmeyer flask with glass stopper. Subsequently, measure 25 ml ethanol abs. in a graduated cylinder, add to the acetic acid, close with the glass stopper, and mix thoroughly. - Reagent 1:
Dissolve 1.0 g of potassium iodide for analysis in 100 ml water for analysis. The solution is stable for about one year protected from light in a tightly closed container at room temperature. - Reagent 2:
In a closed vessel (e.g. 16-mm cells with screw cap) stir 0.025 g 3,3’-dimethylnaphthidine in 5.00 mL of the acetic acid-ethanol mixture [1 + 1] at room temperature (20–25 °C) for 30 minutes (e.g. magnetic stirrer). Should the solution be cloudy or show a bottom sediment it must be filtered with a glass fiber syringe filter (SLAP02550*).
Reagent 2 is stable for 4 weeks if stored and protected from light in closed containers.
* The above-mentioned filter has been checked for interferences. If another filter is used, interferences can occur. Avoid using filters on a cellulose acetate basis.
4. IMPORTANT NOTES
- All used vessels and glassware must be clean and free from surfactant residues or similar substances. Pre-treat the vessels and glassware with a mixture of isopropyl alcohol and hydrochloric acid, if necessary. Subsequently, rinse thoroughly with distilled water.
Prepare the isopropyl alcohol/hydrochloric acid mixture by placing 3 parts isopropyl alcohol in a glass beaker and slowly adding 1 part of hydrochloric acid 25%. Follow the respective safety regulations! - Due to the very sensitive measurement, it is important to work with accurate pipettes throughout the whole analysis.
- The method is strongly affected by temperature. We recommend tempering the reagents and sample at 25°C before preparing the measurement solution and during the reaction time. for details see chapter 10).
- Due to the low bromate concentration, which is accepted as the limit for drinking water (according to WHO and EU Directive 10 μg/L), we recommend checking the recovery rate of the method by means of a bromate standard, especially if new batches of 3,3‘-Dimethylnaphthidine or new preparations of Reagent 2 were used. In case of significant deviations, please recalibrate the method.
Instruction for the recalibration can be found in chapter 12 “User-defined calibration”.
5. Bromate ULR 0.5 – 20.0 μg/L (Prove 600 only)
- 5.1. Sample preparation
- 5.2. Preparation of the measurement solutions
- 5.3 Measurement
6. Bromate ULR 1.0 – 40.0 μg/L
- 6.1. Sample preparation
- 6.2. Preparing the measurement solutions
- 6.3. Measurement
7. Bromate LR 2.5 – 100.0 μg/L (Prove 600 only)
- 7.1. Preparation of the measurement solutions
- 7.2. Measurement
8. Bromate LR 5.0 – 200.0 μg/L
- 8.1. Preparation of the measurement solutions
- 8.2. Measurement
9. Influences of foreign substances
11. Analytical Quality Assurance
- 12.1. User-defined calibration of the Bromate ULR method
- 12.2. User-defined calibration of the Bromate LR method
13. CONCLUSION
The determination of the bromate content of water is a challenge that can be met either by using an elaborate ion-chromatography system or a photometric method. Both approaches demand careful handling and a critical appraisal of the result that is yielded. If the necessary working steps are carefully followed, photometry constitutes a true alternative to chromatography for those laboratories that do not possess ion-chromatography systems.4
For further application notes, see SigmaAldrich.com/wfa-applications.
REFERENCES
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